Electron multiplier



Feb. 14, 1939. E. G. RAMBERG ELECTRON MULTIPLIER Filed Oct. 28, 1936 3nnentor Gttorneg Patented Feb. 14, 1939 UNITED STATES PATENT OFFICE ELECTRON MULTIPLIER tion of Delaware Application October 28, 1936, Serial No. 107,955

9- Claims.

This invention relates to electric discharge devices and particularly to devices of the type wherein amplification of a primary electron stream, such, for example, as is emitted from a thermionic cathode or from a. photosensitive surface exposed to light, is accomplished through utilization of the phenomenon of secondary emission.

If an electrode is subjected to electron bombardment, secondary electrons will be liberated. The ratio of the number of secondary electrons to the number of primary electrons will depend, in part, upon the velocity of impact and upon the nature of the electrode surface material upon which the primary electrons impinge. This ratio can be considerably greater than unity. If the secondary electrons, in turn, are caused to impinge with sufiicient velocity upon a further electrode having a suitably treated surface, the ratio of secondary emission. from the second multiplying electrode may also be greater than unity. If the newly liberated electrons are thrown against another emissive surface, the number of electrons may once more be increased. Hence, one is able to obtain with n multiplying electrodes, in cascade, for example, an amplification of the original or primary electron current equivalent to the amplification per electrode raised to the nth power, provided only that the electrons from one electrode are focused without substantial numerical diminution upon the next succeeding multiplying or target electrode.

Various means have heretofore been proposed for focusing or otherwise directing electrons from one electrode to another in multiplier tubes. Thus, Slepian (U. S. Patent 1,450,265) provides an accelerating electrode for each multiplying electrode and a magnet suitably positioned exterior the tube, so that, when the magnet and electrodes are suitably energized, electrons which otherwise would be drawn to the accelerating electrodes are caused to travel in trochoidal or cycloidal paths and to fall upon the next adjacent multiplying electrode. While electron-multipliers of the described magnetic-type have received wide favor, it may be said generally that the magnet renders the device cumbersome, and further, may introduce disturbing electrical efiects upon associated circuits and apparatus.

Morton and Flory (application Serial No. 6,830, filed February 28, 1935) and McGee (British Patant 443,777 obviate the use of a magnet by the provision of an electron-lens intermediate each pair of multiplying electrodes, whereby the electrons are subjected to a focusing action similar complicated and presents somewhat complex 5 problems of electrode structure and lens design.

Attempts to provide an electron-multiplier ope able without an auxiliary magnet or electronlens system have heretofore not met with a great degree of success, principally because in trying 10 to accelerate the electrons from a source toward the next succeeding multiplying electrode, a portion of the electrons will miss the electrode and impinge on a multiplying electrode beyond that on which it is desired that they should impinge.

(See French Patent 582,428, Dapsence, et al.) As a consequence, the amplification is lowered. The provision of auxiliary plates for physically guiding the electrons along predetermined paths in such tubes complicates their construction without a proportionate increase in over-all efilciency. (See Jarvis, et al., 1,903,569.)

A principal object, therefore, of the present invention is to provide an efficient electron-multiplier operable without the use of auxiliary magnets, electron-lens systems or interposed auxiliary guiding electrodes.

Another object of the invention is to provide an electron-multiplier characterized by an economy of parts and operating potentials.

Other objects and advantages will be apparent and the invention itself will be best understood by reference to the following specification and to the accompanying drawing, wherein Fig. 1 is a view in perspective of a embodiment of the invention, a portion of the tube envelope being broken away to show the elements more clearly,

Fig. 2 is a diagrammatic View of the device of Fig. 1, exemplifying the manner in which the several electrodes are energized when the device is utilized for certain of the purposes to which it is adapted, and

Fig. 3 is a diagrammatic view of a grid-controlled, thermionioally actuated electron-multiplier constructed and operated in accordance with the principle of the invention.

Like characters represent the same or corresponding parts in all figures.

The present invention contemplates and its practice provides an electron-multiplier of the electrostatic type wherein, by reason of certain correlated spacing and dimensions of the electrodes, and of the potentials to which said electrodes are subjected, the electrons are constrained 5 preferred 35 to follow a predetermined inter-electrode path without substantial losses due to the electrons skipping or missing their targets.

More specifically stated, an improved electric discharge device within the invention may be constituted by an evacuated container, preferably, though not necessarily, cylindrical, wherein are disposed a plurality of sets of discrete multiplying electrodes, the electrodes lying in spacedapart planes parallel to each other and to the long or major axis of the container. The electrodes constituting each set are spaced from each other a distance substantially equal to three-fourths the length of a single plate measured along the major axis of the container. Stated another way: the opposite sets of electrodes are spaced from each other a distance substantially equal to three-fourths the distance between corresponding points on adjacent electrodes of the same set. The electrodes of one set are preferably arranged in staggered relation with respect to the electrodes of the other set, that is to say, the space between two electrodes of a given set should preferably be directly opposite the center of an electrode of the other set. The electrodes, if desired, may be solid metallic plates. It is preferable, however, to form them partly of foraminous material, as such construction facilitates the application of the emissive material during construction of the device.

The terminal electrodes, i. e., the primary-electron emitter (or cathode) and the collector electrode (or anode), preferably present so much of their surfaces in planes perpendicular to the planes of the other electrodes that the otherwise open ends of the electrode assembly. are substantially closed to ensure a desired distribution of the electrostatic field present when the device is energized. Considering the cathode to be maintained at ground potential and the electrode upon which primary electrons from the cathode are to impinge to be maintained at +IV volts with respect to the cathode, then to ensure optimum performance each succeeding electrode, in point of electron travel, should preferably be maintained at a potential corresponding respectively to the mathematical series +2V, +3V, +4V, +5V, etc.

Fig. 1 shows an electron-multiplier constructed in accordance with the invention and contained in an elongated evacuated tube T. A pair of oppositely-located, parallelly-arrangedstrips M, M of mica or other suitable insulating material project outwardly from the stern S of the tube and constitute a supporting structure for a set of lower electrodes, numbered I, 3, 5 and I, respectively, and a set of upper electrodes which are designated 2, 4 and B, respectively.- These electrodes are preferably formed of silver and coated with an alkali metal to render them secondarily emissive. Each electrode is provided with two or more lugs n for attachment to the supporting strips M, and with a conductive rodlike lead, I l to I1, respectively, extending through the stem S to the exterior of the tube.

The electrodes are each of the same dimensions, and in the interest of economy, of substantially duplicate construction. In the illustrated embodiment, the electrodes each have an imperforate surface section a and a foraminous or'gridlike section b.

Electrodes 2, 4 and 6 have their surfaces arranged in a common plane with their grid-like surface sections b each directly opposite a solid or imperforate section a of one of the lower electrodes. Section a of electrode l is a photosensitive primary-electron emitting cathode. It is adapted to be actuated by light from an external source L (Fig. 2) positioned to shine through the perforate portion b of electrode 2, opposite thereto. The light supplied by source L may be steady or fluctuating in character.

Section b of cathode I is bent in a plane normal to the electron emitting surface a, whereby to effectively close the otherwise open end of the electrode assembly. The outer edges of electrodes I and 2 do not touch. The lower electrode 1, nearest the stem, is the anode; its imperforate portion a extends towards but does not touch electrode 6, and efiectively closes this end of the assembly. It is upon this section a. that the electrons are eventually collected.

With the ends of the assembly closed in the manner described, the inner transverse edge of each of these terminal electrodes I and 1 falls along a line midway between the corresponding edges of the terminal multiplying electrodes 2 and 6, respectively. With the electrodes thus arranged, the electrostatic field adjacent the ends of the assembly will correspond substantially to that obtaining adjacent and between the central electrodes (34), so that the electrons emitted from corresponding pointson the several electrodes will travel similar paths to their respective targets.

As previously set forth, the electrodes are all of the same dimensions. The common plane of the inner surfaces of electrodes 2, 4 and 6 is spaced from the common plane of the inner surfaces of electrodes 1, 3, 5 and I a distance corresponding substantially to three-fourths the length, measured along the long axis of the tube, of a single electrode. Thus, in one successfully operated device, the plates were each substantially one inch square and the planes of their surfaces were spaced The electrodes in each set were separated approximately from the next adjacent electrode in the same set.

In manufacturing the device of Fig. 1, the electrodes were first assembled upon the mica strips M, M, and the leads H to I! welded to electrodes I to I respectively. The entire assembly was then mounted within the tube T which was then heated and evacuated. After evacuation, oxygen was introduced into the tube at a pressure in the neighborhood of 1 mm. of mercury, and the gas activated to cause oxidization of the silver surfaces of the several electrodes.

After the electrodes were oxidized, the residual oxygen was pumped out of the tube and alkali metal, in this case caesium, distilled into it. By reason of the openings in the grid-like portions b of the electrodes, the caesium vapor entered readily into the interior of the assembly to bathe the inner, opposed, electrode surfaces. The tube was next baked for about ten minutes at a temperature of 210 C. to cause the alkali metal to combine with the silver oxide, thus giving rise to a highly photo-sensitive and secondarily-emissive surface. During the baking step the excess caesium was pumped out of the tube. The tube was then sealed off.

As heretofore mentioned, and in accordance with the invention, the potential distribution among the electrodes, required to ensure optimum performance, may be expressed by the mathematical series IV, 2V, 3V, 4V, 5V, 6V, etc., where IV is the potential drop between the primary electron source and the first target electrode, and 2V, 3V, V, etc., represent the potential drop b tween the respective 1 succeeding electrodes, in point of electron travel, and said source.

Referring now to Fig. 2. For the purpose of providing such a potential distribution, the oathode I may be connected to the negative terminalof a source of unidirectional potential, exemplified in the drawing by a resistor R and the first multiplying electrode, i. e., the electrode 2, whose surface is opposite the photo-sensitive cathode, connected to a point IV somewhat more positive. The other electrodes 3 to 1, in the order of their numbers, are shown connected to successively more positive points, 2V to 6V, respectively, on the resistor.

The reference characters IV, 2V, 3V, 4V, etc., given to the several points on resistor R, will be understood to indicate that the voltage drop between a given electrode and the cathode is the designated whole number multiple of the voltage drop existing between the cathode I and the first multiplying electrode 2. Thus, in a device of the previously described construction, where the potential drop between the first multiplying electrode 2 and cathode I is 100 volts, the drop between electrodes 3 and I should be 200 volts, that between electrodes 4 and I, 300 volts.

If a beam of light, say of varying intensity, is caused to fall upon the first lower electrode I, photo-electrons will be emitted in a quantity determined by the instantaneous intensity of the light beam. These photo-electrons will be accelerated toward the upper electrode 2 and because of the described correlation between their size, position and applied voltages will impinge upon the imperforate portion 2a of this first multiplying electrode 2. The photo-electrons striking electrode 2 will cause the emission of secondary electrons; the number of secondary electrons emitted is dependent, in part, upon the magnitude of the potential difference between it and the cathode.

As indicated by the broken line e, the next electrode, in point of electron travel, is the second lower electrode 3. The trajectory of secondaryelectrons from section 2a of the first multiplying electrode is such that they impinge upon section 3a of the second multiplying electrode 3. Here again, a multiplication, by reason of secondary emission, is secured and this process is repeated in any number of stages until the amplified stream of secondary electrons is collected by the solid upright portion of a of the output electrode I and caused to flow in a utilization circuit exemplified in the drawing by the resistor r included between the output electrode TI and the positive terminal 6V of the potential divider.

A thermionic primary electron source, instead of a photo-electric source, may be employed, if desired, for rendering the device capable of uses to which well-known thermionic tubes are put. For example, referring to Fig. 3, an alternative embodiment of the invention is illustrated wherein a controllable electron source is substituted for the photo-sensitive section a of electrode I of Figs. 1 and 2. The container '1 is provided with a depending neck portion t which supports a virtual source of electrons in the plane of the lower multiplying electrodes 3 and 5. The electron source may be constituted by a metallic thimble 2!, the upper end of which has a layer of electron emissive oxides 22. The thimble is completely surrounded and shielded by a cylindrical metallic grid structure 23 which terminates in a perforated cap 24. The upper face of the cap lies in the plane of the multiplying electrodes 3 and 5, and, preferably, the perforation therein is coaxial with the emitting portion of the cathode, and is covered by a fine screen 25 to which it is electrically connected. The grid structure 25, when supplied with properpotentials, either direct or fluctuating, serves tocontrol the -emis sion from the thermionic cathode in the same way as emission from the photo-sensitive cathode I in the device of Figs. 1 and 2 is caused to be controlled by variations in the light impinging thereon.

As will be clearly apparent from a close inspection of Fig. 3 of the drawing, the potential distribution on the various electrodes and the cathode of the tube may be the same as exemplified by Fig. 2. Obviously, any desired input circuit elements may be connected between the grid and the cathode of the tube, exemplified in the drawing by an input resistor 26, a grid biasing potential source 21 and a potential divider 23.

To simplify the drawing, only five multiplying stages are shown in the described embodiments of the invention. It is to be understood, however, that as many stages as desired may be incorporated into the device, provided only that the highest potential required to operate the device (in accordance with the formula of the invention) is not so great as to cause interelectrode arcing.

The external potential-dividing device exemplified in the drawing by the resistor B. may be dispensed with, if desired, by the provision of individual resistors connecting adjacent electrodes in the same plane. Such resistors (not shown) may be quite small and, consequently, they may be built into the electrode assembly itself, for instance in the manner described in copending application Serial No. 48,982 to Zworykin and Massa.

Other modifications of the invention will be apparent to those skilled in the art. It is to be understood, therefore, that the foregoing is to be interpreted as illustrative and not in a limiting sense, except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

1. An electric discharge device comprising an evacuated tube having a major axis, an anode and a cathode mounted in said tube, and a plurality of sets of multiplying electrodes mounted in staggered relation in spaced parallel planes on opposite sides of said major axis intermediate said cathode and anode, said multiplying electrodes each having substantially the same length measured along said major axis, the distance between the planes of the electrodes of each set being substantially equal to three-quarters of the length of a single of said multiplying electrodes.

2. An electron multiplier comprising a bi-part electrode constituted of a continuous surface partly of grid-like and partly of imperforate construction, said imperforate surface section being electron-emissive, and a second electrode of duplicate construction mounted in a plane parallel to said first-mentioned electrode and with its imperforate surface section accessible to electrons from said first-mentioned electrode,

3. The invention as set forth in claim 2 wherein the imperforate surface portion of one electrode is opposite the grid-like surface portion of the other of said electrodes.

4. An electron multiplier comprising a plurality of sets of opposed electrodes mounted on opposite sides of a common axis, electrode lead for establishing an electrostatic field between said opposed electrodes, and means constituting a portion of a terminal electrode substantially enclosing an otherwise open terminal end of said electrode assembly for ensuring a desired distribution of said electrostatic field.

5. The invention as set forth in claim 4 wherein said terminal electrode is an electron-emitting cathode.

6. The invention as set forth in claim 4 wherein said terminal electrode is an electron-collecting anode.

'7. The invention as set forth in claim 4 wherein each of the terminal ends of said electrode assembly is enclosed by a portion of a terminal electrode.

8. An electron multiplier comprising a cathode, an anode, and a plurality of sets of multiplying electrodes mounted in spaced relation on opposite sides of an axis which extends between said cathode and anode, the distance between said sets of multiplying electrodes measured across said axis being substantially equal to three-quarters of the distance between corresponding points on adjacent electrodes on the same set, the electrodes of one set being ofiset in the anode direction from the electrodes of another set.

9. An electron-multiplier comprising a cathode, an anode and a plurality of sets of multiplying electrodes mounted in staggered relation on opposite sides of an axis which extends between said cathode and anode, said multiplying electrodes each having substantially the same surface dimension measured along said axis, the distance between said opposed sets of multiplying electrodes as measured along a line substantially normal to said axis being less than the said surface dimension of said multiplying electrodes.

- EDWARD G. RAMBERG. 

